When it comes to designing the moving structural elements of high-performance machine tools—such as beams, shafts and spindles, and the assemblies used to hold cutting tools—the need for the machine tool to be accurate, consistent and durable translates into being built of parts that are light, strong, and stiff. Whichever material a designer selects, the physical requirements call for hollow section members with thin walls.
Thin-walled hollow structures have the stiffness and low weight, but they can have other characteristics that lower machine performance. For example, if the natural frequency of a beam or its walls is too close to the machine’s operating frequencies, the resulting resonance can cause significant vibrations that impair accuracy and could lead to the machine becoming unstable and perhaps dangerous. A machine may also have to operate across a wide temperature range due to changes in the weather or heat from the workpiece or heat generated during cutting, machining or welding. Over time, the resulting thermal expansion and contraction can lead to significant dimensional changes.
The Composite Advantage
Appropriately designed and manufactured carbon fiber-reinforced composite components can overcome many of the major challenges facing machine tool designers. Carbon composite elements can be as little as a quarter of the mass of steel ones and have the same strength and stiffness. A unidirectional carbon fiber reinforced laminate can deliver twice the stiffness of steel in the direction of the fibers, thanks to the properties of graphite fiber. They can be engineered with vibration-damping properties 20 times better than steel or for zero expansion in one dimension.
A further advantage of composites is the opportunity to tune the part’s properties to suit specific requirements of each application. This is done by adjusting the quantity and orientation of fibers in the structure. It can also be done sometimes by incorporating other materials or components into the part while it is being made. Designing a composite component for the best overall performance in each machine application requires understanding the end-user’s requirements and the pros and cons of various composite manufacturing techniques.
For example, axial fiber placement lets fibers be aligned along the length of the tube or beam. In this manufacturing technique, an array of radial pins is fitted to each end of the mandrel used to create the part. Fibers get wrapped around these pins, laid along the length of the mandrel, and are then wrapped around the corresponding pin at the other end. Repeating this process around each pin builds a complete layer of longitudinal fibers over the part’s surface. This provides ideal fiber alignment for bending loads. Axial fiber placement allows for the creation of composite parts with a very high percentage of fibers within their structure. Compared to conventional filament winding techniques, axial fiber placement can produce beams and tubes that are 10% to 15% stiffer in the direction of the fiber and offer 50% greater strength in bending.
Reducing Vibrations
One area of focus in designing machines is fine-tuning the vibration-damping properties. The low mass and high stiffness of carbon composite parts already provides good vibration and damping characteristics compared to steel or aluminium alternatives. Vibrations are so important in machine applications, however, that they require special attention during design.
Vibration behaviour can be handled several ways. The part’s dimensions and wall thickness can be adjusted to tune its natural frequencies, for example. Damping materials, including rubber and cork fillers, can be incorporated into the structure. Internal foam reinforcements may be included to improve vibrational stability of the walls in large parts.
One manufacturer of laser cutters used this approach to significantly improve a new machine’s performance. The company was trying to reduce weight and deflection in a Y-axis transverse beam that was over three feet long. It replaced the existing steel part with a thin-walled composite part reinforced with foam. This reduced beam weight by 44% while increasing its stiffness. Those improvements let the beam’s peak acceleration go from 3g to 6g. That, in turn, reduced the time to cut a sheet of material in the machine by up to 30%. Furthermore, the part’s extra stiffness and improved damping characteristics can improve accuracy by up to 50%.
Making it All Work
Regardless of their inherent characteristics, composite parts must work effectively in the overall structure of the machine. That means they must accommodate appropriate mounting points for ancillary equipment, brackets for axles and bearings, and tracks for other moving components. Each of these elements must be strong and stable enough for reliable operations.
To more easily get composite parts to work in machines, appropriate mounting points can be built into them when they’re made. These might include additional layers of machinable material to accomodate the precision surface for linear rails, installing metal inserts in the composite structure to allow threaded holes for connections or adding pads to support tracks and brackets.
A good example of this approach is a novel transfer beam. During the design process, the concept went from a basic square-section beam into a hybrid composite version of a steel T-slot beam. Incorporating the T-slot in the composite beam during manufacture let the company use standard component mounts, retaining all the operational flexibility of the original design. The change led to significant performance improvements, however. The new composite beam was stiffer than steel predecessor and had only a quarter of its mass. As a result, the part’s natural frequency increased while its dynamic response decreased. The amplitude of steel beam’s displacement limited the press’s line speed, but the composite beam let the operating rate increase from 20 to 32 strokes per minute, giving an average output increase of 40%.
Costs
As a raw material, carbon fiber is significantly more expensive than steel or aluminium. That fact alone is enough to stop some machine designers from considering composite parts. It is important, however, to take a holistic approach when comparing costs of the composite parts.
Composite manufacturing can turn out accurate components with little post-processing, and this significantly narrows the cost gap between metal and composites by the time a part is readied to be installed on a machine. In some cases, composites can be a lower-cost option than metal counterparts.
Even when, as is more common, the cost of the composite beam is slightly greater than the alternative, it often generates savings elsewhere. Reducing the weight of major moving components lets designers use smaller bearings, motors and other motion components. The reduction in weight and, therefore, energy will pay back the extra cost well within the machine’s lifetime. Lower weight also reduces momentum forces and can extend the life of these same parts that will wear over time. This means serve and replacement periods can be lengthened.
In most applications, however, the most important benefits of carbon composite machine parts arise from improvements in speed, throughput and quality. High-performance machines make money for their users, and the annual productivity payback from using composite parts can be tens or hundreds of times greater than any one-time additional cost.
Humphrey Carter is head of business development at Compotech. His email is [email protected].